The present invention relates to a catalyst, an exhaust gas purifying catalyst, a method for purifying exhaust gas and a method for preparing a catalyst.
Gas exhausted from an engine of a car or the like includes HC (hydrocarbon), CO and NOx (nitrogen oxides). A known catalyst for purifying the exhaust gas is a ternary catalyst for simultaneously oxidizing HC and CO and reducing NOx. Also, it has been proposed to dispose a HC adsorbent of zeolite in an exhaust passage in the upstream side of the ternary catalyst, so as to adsorb HC included in the exhaust gas when the ternary catalyst has low activity such as immediately after start-up of the engine. Such a HC adsorbent, however, tends to start discharging HC before the ternary catalyst becomes active, and in this case, even though the HC included in the exhaust gas is adsorbed, the HC is discharged to the air without purification.
As a countermeasure, Japanese Laid-Open Patent Publication No. 7-256114 discloses that the adsorbing performance of a HC adsorbent (zeolite) is improved when Ag is carried on the HC adsorbent and that the heat resistance of the Ag can be improved when Bi is further carried on the HC adsorbent.
Furthermore, this publication describes that a HC adsorbent layer in which Ag and Bi are carried on a HC adsorbent and a ternary catalyst layer in which a noble metal selected from Pt, Pd and Rh is carried on alumina and ceria are supported on a honeycomb carrier with the HC adsorbent layer disposed inside. Thus, HC discharged from the inside HC adsorbent layer can be oxidized by the outside ternary catalyst layer.
This catalyst is prepared as follows: A powder in which Ag and Bi are carried on zeolite is supported on the honeycomb carrier by a wash coat method, so as to form the HC adsorbent layer. Then, a powder in which Pt is carried on ceria is supported on the HC adsorbent layer by the wash coat method, so as to form a Pt catalyst layer. Thereafter, a powder in which Rh is carried on alumina is supported on the Pt catalyst layer by the wash coat method, so as to form a Rh catalyst layer. According to the publication, Pt may be replaced with Pd.
In the catalyst including Ag and Pd, however, although Pd naturally has high activity as an oxidation catalyst at a low temperature, the temperature at which its activity starts to be exhibited is disadvantageously increased due to the presence of Ag. According to the study of the present inventor, this is because Pd and Ag are reacted to each other to produce a Pd—Ag alloy, which increases the activation temperature. Also, even when Bi is disposed in the vicinity of Ag by allowing zeolite to carry both Ag and Bi, the degradation of the low temperature activity of Pd cannot be avoided.